Engine exhaust and in particular, diesel engine exhaust, is a heterogeneous mixture which contains not only gaseous emissions such as carbon monoxide (“CO”), unburned hydrocarbons (“HC”), and nitrogen oxides (“NOx”), but also condensed phase materials (liquids and solids), commonly referred to as particulates or particulate matter (PM). Regulated species of exhaust emissions include CO, HC, NOx, and PM.
Often, catalyst compositions and substrates on which the compositions are disposed are provided in diesel engine exhaust systems to convert certain or all of these exhaust components to innocuous components. For example, diesel exhaust systems can contain one or more of a diesel oxidation catalyst, a soot filter and a catalyst for the reduction of NOx.
Oxidation catalysts that contain platinum group metals, base metals, and combinations thereof are known to facilitate the treatment of diesel engine exhaust by promoting the conversion of both HC and CO gaseous pollutants and some proportion of the particulate matter through oxidation of these pollutants to carbon dioxide and water. Such catalysts have generally been contained in units called diesel oxidation catalysts (DOCs), which are placed in the exhaust of diesel engines to treat the exhaust before it vents to the atmosphere. In addition to the conversion of gaseous HC, CO and particulate matter, oxidation catalysts that contain platinum group metals (which are typically dispersed on a refractory oxide support) also promote the oxidation of nitric oxide (NO) to NO2.
The total particulate matter (TPM) emissions of diesel exhaust are comprised of three main components. One component is the dry, solid carbonaceous fraction or soot fraction. This dry carbonaceous matter contributes to the visible soot emissions commonly associated with diesel exhaust. A second component of the particulate matter is the soluble organic fraction (“SOF”). The soluble organic fraction is sometimes referred to as the volatile organic fraction (“VOF”), which terminology will be used herein. The VOF can exist in diesel exhaust either as a vapor or as an aerosol (fine droplets of liquid condensate) depending on the temperature of the diesel exhaust. It is generally present as condensed liquids at the standard particulate collection temperature of 52° C., in diluted exhaust, as prescribed by a standard measurement test, such as the U.S. Heavy Duty Transient Federal Test Procedure. These liquids arise from two sources: (1) lubricating oil swept from the cylinder walls of the engine each time the pistons go up and down; and (2) unburned or partially burned diesel fuel. The third component of the particulate matter is the so-called sulfate fraction. The sulfate fraction is formed from small quantities of sulfur components present in the diesel fuel and lubricating oil. During combustion, the sulfur components of the diesel fuel and oil form gaseous SO2 and SO3. As the exhaust cools, SO3 combines rapidly with water to form sulfuric acid, H2SO4. The sulfuric acid forms an aerosol that collects as a condensed phase with the carbon particulates, or is adsorbed onto the other particulate components, and thereby adds to the mass of TPM.
One key after-treatment technology in use for high particulate matter reduction is the diesel particulate filter (DPF). There are many known filter structures that are effective in removing particulate matter from diesel exhaust, such as honeycomb wall flow filters, wound or packed fiber filters, open cell foams, sintered metal filters, etc. However, ceramic wall flow filters, described below, receive the most attention. These filters are capable of removing over 90% of the solid carbonaceous particulate material from diesel exhaust. The filter is a physical structure for removing particles from exhaust, and the accumulating particles will increase the back pressure from the filter on the engine. Thus the accumulating particles have to be continuously or periodically burned out of the filter to maintain an acceptable back pressure. Unfortunately, the carbon soot particles require temperatures in excess of 500° C. to burn under oxygen rich (lean) exhaust conditions. This temperature is higher than that typically present in diesel exhaust.
Provisions are generally introduced to increase exhaust temperature in order to provide for active regeneration of the filter. The presence of a catalyst provides for CO, HC and NO oxidation within the filter and an increase in the rate of soot combustion. In this way a catalyzed soot filter (CSF) or catalyzed diesel particulate filter (CDPF) is effective in providing for >90% particulate matter reduction along with active burning of the accumulating soot.
Another mechanism for the removal of particles is through the use of NO2 in the exhaust stream as an oxidant. Thus, particulates may be removed by oxidation employing NO2 as an oxidant at temperatures above 300° C. The NO2 already in the exhaust from the engine may additionally be supplemented through oxidation of NO (also in the exhaust) through the use of an upstream DOC oxidation catalyst. This passive regeneration mechanism can further reduce the soot load in a filter and decrease the number of active regeneration cycles.
Future emissions standards adopted throughout the world will also address NOx reductions from diesel exhaust. A proven NOx abatement technology applied in heavy-duty mobile emission systems since 2006 in Europe and since 2010 in US with lean diesel exhaust conditions is Selective Catalytic Reduction (SCR). In this process, NOx is reduced with ammonia (NH3) to nitrogen (N2) over a catalyst typically composed of base metals. The technology is capable of NOx reduction greater than 90%, and thus it represents one of the best approaches for achieving aggressive NOx reduction goals. SCR for mobile applications uses urea (typically present in an aqueous solution) as the source of ammonia. SCR provides efficient conversions of NOx as long as the exhaust temperature is within the active temperature range of the catalyst.
While separate substrates, each containing catalysts to address discrete components of the exhaust, can be provided in an exhaust system, use of fewer substrates is desirable to reduce the overall size of the system, to ease the assembly of the system, and to reduce the overall cost of the system. One approach to achieve this goal is to coat the soot filter with a catalyst composition effective for the conversion of NOx to innocuous components (giving an “SCR-catalyzed soot filter” or “coated soot filter”). With this approach, the SCR-catalyzed soot filter assumes two catalyst functions: removal of the particulate component of the exhaust stream and conversion of the NOx component of the exhaust stream to N2.
Coated soot filters that can achieve NOx reduction goals require a sufficient loading of SCR catalyst composition on the soot filter. The gradual loss of the catalytic effectiveness of the compositions that occurs over time through exposure to certain deleterious components of the exhaust stream augments the need for higher catalyst loadings of the SCR catalyst composition. However, preparation of coated soot filters with higher catalyst loadings can lead to unacceptably high back pressure within the exhaust system. Coating techniques that allow higher catalyst loadings on the wall flow filter, yet still allow the filter to maintain flow characteristics that achieve acceptable back pressures are therefore desirable.
An additional aspect for consideration in coating the wall flow filter is the selection of the appropriate SCR catalyst composition. First, the catalyst composition must be durable so that it maintains its SCR catalytic activity even after prolonged exposure to higher temperatures that are characteristic of filter regeneration. For example, combustion of the soot fraction of the particulate matter often leads to temperatures above 700° C. Such temperatures render many commonly used SCR catalyst compositions such as mixed oxides of vanadium and titanium less catalytically effective. Second, the SCR catalyst compositions preferably have a wide enough operating temperature range so that they can accommodate the variable temperature ranges over which the vehicle operates. Temperatures below 300° C. are typically encountered, for example, at conditions of low load, or at engine startup. The SCR catalyst compositions are preferably capable of catalyzing the reduction of the NOx component of the exhaust to achieve NOx reduction goals, even at lower exhaust temperatures.
There remains a need in the art for catalyst articles, methods and systems to treat the carbon monoxide, nitrogen oxides, hydrocarbons and particulate matter from a diesel engine in an effective and inexpensive manner while simultaneously minimizing required space in the exhaust system.